This invention concerns a process and a catalyst for producing very pure olefins of the formula ##STR3## from the corresponding ethers of the formula ##STR4## where R, R.sub.1, R.sub.2 and R.sub.3, identical or different, are alkyl, arylalkyl, aryl or alkylaryl radicals, R.sub.2 and R being optionally a hydrogen atom.
It is known that the olefinic hydrocarbons, when contacted with an acid such as sulfuric acid or with a solid having appropriate acidic properties, react with alcohols to produce ethers. The velocity of this reaction depends on the operating conditions (pressure, temperature, contact time, etc. . . . ) used for carrying out this reaction.
A judicious choice of these operating conditions makes it possible to effect the selective reaction, in an olefinic charge, of the olefins of formula ##STR5## (i.e. having a tertiary carbon atom in the immediate vicinity of the double bond), with at least one primary alcohol of formula R--CH.sub.2 OH, so as to form tert-alkyl ethers, according to reaction (1) below.
These tert-alkyl ethers may in turn be decomposed, in the presence of a catalyst and under suitable operating conditions, to give, selectively, the alcohol and the starting olefin, also called tertiary olefin in the literature.
These two successive reactions have been used in the prior art for producing so-called tertiary olefins from an olefinic cut: Thus, processes for manufacturing tertiary olefins (isobutene, isopentene, etc. . . . ) of high purity, have been described, in the literature, based on the selective "extraction" of the tertiary olefins from an olefinic cut, i.e. the treatment of the tertiary olefin with an alcohol, taking advantage of the selectivity of reaction (1), whereby it is possible to obtain the corresponding tertiary ether: ##STR6##
The desired so-called tertiary olefin is thus separated in the form of an ether, which is easily isolated, for example by distillation of the mixture withdrawn from the reaction zone, which mixture contains unreacted hydrocarbons, the excess of alcohol (generally a primary alcohol) and the formed ether.
The final step of the process is then the decomposition of the obtained ether into its starting constituents: the alcohol and the desired tertiary olefin. The alcohol may be recycled in order to be again reacted with the tertiary olefins of the fresh cut used as charge. Such a decomposition step is described, for example, in the U.S. Pat. No. 3,170,000, in the presence of a catalyst whose specific surface must be lower than 25 m.sup.2 /g, and in the British Pat. No. 1,176,620 wherein the operation is conducted in the presence of steam.
However, this type of decomposition reaction, up to now, was not sufficiently selective as a result of parasitic reactions taking place therewith and which decrease the yields of tertiary olefin and of recovered alcohol.
These parasitic reactions are particularly of the following types:
the formed alcohol may react with itself while dehydrating, to produce an ether and water (for example dimethylether and water when the alcohol is methanol); PA1 the obtained olefin may dimerize, or even trimerize; PA1 the olefin may also be hydrated to the corresponding tertiary alcohol.
In view of the fact that the desired tertiary olefins form a very valuable raw material for the manufacture of derivatives (for example gasolines, alkylates, polymers and other chemical products), it is thus very important to obtain them in their highest possible degree of purity.
The catalysts used up to now for this reaction of decomposing tert-alkyl-ethers are not sufficiently selective since, in most cases, they favour the formation of dialkyl ethers by dehydration of the corresponding primary alcohols which have been formed. This parasitic reaction is the more favoured as the reaction temperature is the higher; in this connection it must be mentioned that many of the catalysts used require relatively high temperatures in order to have a sufficient activity.
This parasitic reaction results in a loss of alcohol and, consequently, in the requirement of fresh alcohol addition to the recycled alcohol supplied to the reactor where the initial etherification reaction of the tertiary olefin is effected.
Another disadvantage in connection with the production of dialkylether is the necessity of using a more complete distillation installation, in view of the fact that this "undesirable" ether has to be separated from the desired tertiary olefin.